A PropertySource implementation capable of interrogating its
underlying source object to enumerate all possible property name/value
pairs. Exposes the getPropertyNames() method to allow callers
to introspect available properties without having to access the underlying
source object. This also facilitates a more efficient implementation of
containsProperty(String), in that it can call getPropertyNames()
and iterate through the returned array rather than attempting a call to
PropertySource.getProperty(String) which may be more expensive. Implementations may
consider caching the result of getPropertyNames() to fully exploit this
performance opportunity.

Most framework-provided PropertySource implementations are enumerable;
a counter-example would be JndiPropertySource where, due to the
nature of JNDI it is not possible to determine all possible property names at
any given time; rather it is only possible to try to access a property
(via PropertySource.getProperty(String)) in order to evaluate whether it is present
or not.